The present invention relates to integrated circuits and, more particularly, to a non-volatile variable resistive element which may be deposited on the surface of an integrated circuit.
The ability to construct large numbers of small variable resistors on the surface of an integrated circuit would be very advantageous. For example, so called "neuro-networks" can require hundreds of thousands of resistors. If the neuro-network is to be programmable, each of these resistors must be capable of being addressed and the resistance value changed in a continuous manner. Prior art solutions to providing such programmable resistors have been less than ideal.
For example, in one prior art solution, each resistor is constructed from a plurality of fixed resistors and switching transistors. The fixed resistors typically have resistance values which differ from one another by factors of two. The switching transistors are used to connect selected resistors in series to form the variable resistance. The specific resistors are specified by digital signals. In addition, such a system must have addressing circuitry which decodes digital signals specifying the address of the specified resistor and routes the various bits of a binary number to the various switching transistors. Since each of the fixed resistors requires an area of silicon which is larger than an FET, a variable resistor which provides even 128 resistance values requires a silicon area which is larger than that needed to construct 100 FET transistors. Hence, a circuit with 100,000 such resistors on a single chip is expensive to fabricate.
Furthermore, if power is lost, the value of each resistor is also lost. As a result, the chip would have to be reprogrammed each time power is lost. This is time consuming and requires some form of non-volatile storage for the various resistance values.
A second proposed prior art solution for providing such variable resistance elements utilizes electrically erasable read only memory cells (EEPROM) for the resistive elements. In this solution, the EEPROM cells are operated in an analog region. The resistance between the drain and source of each of the cells is determined by the amount of charge on a floating gate. Charge is transferred to and from the floating gate by tunneling mechanisms.
The EEPROM resistive elements require much smaller areas of silicon and are non-volatile; hence, they solve the above mentioned problems of a variable resistor based on fixed resistors. Unfortunately, EEPROM resistive elements have two problems. First, the time needed to program an EEPROM can be of the order of milliseconds. Hence, to program all of the resistors in a large neuro-network can require several seconds. Programming such a network can require reprogramming each resistor as many as thousands of times. Hence, EEPROM based neuro-networks can require prohibitively long programming times.
Second, EEPROM memory devices may only be reprogrammed about 10,000 times before the devices fail. As noted above, EEPROM cells operate by causing electrons to tunnel between a floating gate and some other electrode. The space between the floating gate and tunneling electrode is typically filled with silicon oxide. Some of the electrons become trapped in the oxide during each of the tunneling operations. As a result, a space charge which increases with each reprogramming accumulates. This space charge eventually prevents electrons from tunneling between the floating gate and the tunneling electrode. However, even before this charge builds up, the programming voltage needed to transfer a specified charge to the floating gate changes, making it difficult to predict the change in resistance when a given programming signal is applied to the EEPROM.
As noted above, literally thousands of reprogrammings may be needed during the programming stage of setting up a neuro-network. At each reprogramming, a predictable change in the resistance of the element must occur. As noted above, the EEPROM cell will begin to change after a few thousand reprogrammings. Hence, it is not always possible to program the network before the EEPROM elements wear out.
A third problem inherent in prior art EEPROM cells is the large voltages needed to cause the electrons to tunnel during programming and erasing operations. Voltages of the order of 17 to 25 volts are typically needed. Such voltages require isolation of the circuitry from other low voltage circuitry on the same integrated circuit chip. In addition, special circuitry for generating the high voltage from the normally available low voltage supply must also be included on the chip.
Efforts to overcome these problems utilizing ferroelectric-based materials are well known to those skilled in the art. For example, U.S. Pat. No. 2,773,250 describes a device for storing information in which the device consists of a ferroelectric body having a semiconductor layer deposited thereon. The semiconductor layer acts as a variable resistor in an electric circuit. The resistance of the semiconductor layer is controlled by the degree of polarization of the ferroelectric body. The polarization of the ferroelectric body is controlled by generating an electric field in the ferroelectric body. The electric field is generated by providing a voltages difference across the ferroelectric body. This voltage difference is generated by connecting the semiconductor layer to one voltage and a second programming electrode consisting of a conductive layer which was deposited on the other side of the ferroelectric body to a second voltage.
This resistive element, however, does not function well over long periods of time due to the materials selected. The device in question utilizes a barium titanate ferroelectric and a tellurium semiconductor. Tellurium oxidizes readily at room temperature. Hence, an oxide layer can form between the tellurium layer and the ferroelectric body as oxygen atoms drift from the barium titanate into the tellurium under the influence of the electric fields used to polarize the barium titanate. The oxide layer has a dielectric constant which is much less than that of the barium titanate. Hence, as the oxide layer forms, the voltage difference that must be applied between the programming electrode and the semiconductor to change the polarization of the barium titanate increases. After an unacceptably small number of resistive element progammings, the required voltage becomes too large for the device to be practical.
A second type of EEPROM cell based on ferroelectric materials has been proposed in the prior art. In this type of EEPROM cell, the gate oxide of a field effect transistor (FET) is replaced with a ferroelectric material such as lead lanthium zirconate titanate (PLZT). The material is polarized by placing a voltage difference between the gate of the FET and the source. The polarization gives rise to an effective space charge at the boundary between the gate oxide, and channel region. The magnitude and polarity of this space charge depends on the degree of polarization of the PLZT material and the direction of polarization, respectively. In one direction of polarization, the carrier density in the channel is reduced, leading to an increased resistance between the source and drain of the FET. The resistance value is specified by the polarizing voltage. The time to switch the polarization of the PLZT material is of the order of nanoseconds and the polarization may be switched 10.sup.9 times without damaging the device. Hence the above mentioned problems encountered with tunneling EEPROM cells are avoided.
Unfortunately, this type of PLZT structure is difficult to fabricate and, in practice, may be programmed only a relatively small number of times. In this type of EEPROM the electric field used to deplete the channel region is the remnant electric field resulting from the polarization of the PLZT layer. This electric field is significantly less than the electric field obtained by applying a charge to a floating gate. Hence, the depth of the channel region that can be depleted in response to this electric field is significantly less than that available in normal EEPROM cells. Channels having small depths are difficult to fabricate.
In addition, this device suffers from the same type of material incompatibilities described above with reference to the barium titanate based resistive element. In particular, no satisfactory manner has been found to protect the channel region from the metal ions in the PLZT material. The PLZT material must be crystallized on the silicon substrate at temperatures of 500.degree. C. At these temperatures, the lead ions diffuse into the channel region. These metal ions change the electrical properties of the channel in a manner analogous to doping the channel with metal ions to control its carrier density.
If a barrier such as silicon dioxide is deposited before the PLZT material to protect the channel drifting ions, the programming voltage is increased to an unacceptable value. The EEPROM is normally programmed by applying a voltage between the gate of the FET and the channel region. Systems requiring large programming voltages are very expensive to fabricate. The available barrier materials are insulators with a dielectric constant significantly less than that of the PLZT material. Consider the case in which a voltage is applied between the channel and gate of the EEPROM to polarize the PLZT layer. Part of the voltage will appear across the barrier material and the remainder will be appear across the PLZT material. The fraction appearing across the PLZT layer is determined by the relative dielectric constants of the barrier material and the PLZT material. In general, the PLZT materials have much higher dielectric constants than the available barrier materials. As a result, most of the voltage appears across the barrier material. It has been found that the inclusion of such a barrier increases the programming voltage to more than 100 volts.
Although the direction and magnitude of the remnant polarization of the PLZT material can be altered more than 10.sup.9 times, the observed life-time of this type of EEPROM is less than that of conventional EEPROM cells. The short life-time is the result of a different form of ion drift. The PLZT materials include oxygen atoms which can drift in response to the voltages used to change the remnant polarization in the PLZT layer. As noted above, to alter the remnant polarization, a voltage must be applied across the PLZT layer. This is normally accomplished by applying a voltage between the channel region of the FET and the gate electrode. The magnitude of this voltage is sufficient to cause ions to drift. Depending upon the direction of the applied voltage differences, ions will either drift from the PLZT layer into the channel region or from the channel region into the PLZT layer. When oxygen ions drift from the PLZT layer into the channel region, they form a silicon dioxide layer at the interface between the PLZT layer and the channel region. As noted above, silicon dioxide is an insulator with a low dielectric constant. As a result, the programming voltage slowly increases with time.
Accordingly, it is an object of the present invention to provide an improved programmable non-volatile resistive element.
It is another object of the present invention to provide a resistive element that can be reprogrammed more times than existing variable resistive elements.
It is yet another object of the present invention to provide a resistive element that can be more economically fabricated than resistive elements based on EEPROM cells.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.